The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process fluid temperature transmitter provides an output related to a sensed process substance temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled In order to monitor a process fluid temperature, the transmitter includes a sensor, such as a resistance temperature device (RTD).
An RTD changes resistance in response to a change in temperature. By measuring the resistance of the RTD, temperature can be calculated. Such resistance measurement is generally accomplished by passing a known current through the RTD, and measuring the associated voltage developed across the RTD.
One source of error in RTD thermometry is due to the presence of non-associated voltages present in the RTD circuit. A non-associated voltage is any voltage in an RTD circuit that is not generated by the flow of a measurement current. Examples of non-associated voltages include voltages due to the Seebeck Effect and galvanic action. Reference may be made to the McGraw-Hill Encyclopedia of Science and Technology, seventh edition.
The Seebeck Effect provides that dissimilar metal junctions create junction voltages due to the union of dissimilar metals in a temperature gradient condition. Thus, the voltage measured across the RTD will include not only the associated voltage corresponding to the current passing through the RTD, but also the non-associated junction voltages as well. If compensation for these non-associated voltages is not provided, measurement error can result.
Non-associated voltages due to the Seebeck Effect are present in many known RTD""s. Such RTD""s typically have at least one dissimilar metal junction. For example, the sensing portion of an RTD may be constructed from a winding of platinum sensor wire that is electrically coupled to two or more leadwires. The leadwires are generally copper, and can often be plated with nickel. Thus, each junction of sensor wire to leadwire can include a platinum-nickel junction and a nickel-copper junction. Additionally, in some RTD""s it is difficult to couple the sensor wire directly to the leadwire and so a transition coupling is used. The transition coupling is often a gold-plated tab to which both the sensor wire and the leadwire are attached. Use of the transition coupling adds additional dissimilar metal junctions. Further, dissimilar metal junctions can also occur where the leadwires couple to transmitter terminals. In some instances such non-associated voltage error can approach 0.5% which is larger than the desired accuracy of 0.1xc2x0 C. over a 1000xc2x0 C. span.
Non-associated voltages can also be created by galvanic action within an RTD circuit. For example, in some RTD""s the sensing wire is wound around a ceramic core that provides and insulating support for the sensor wire. However, due to impurities in the ceramic, the core will allow some current flow and can thus create an emf (electromotive force) that is a non-associated voltage within the RTD circuit. The error caused by galvanic action can be as large or greater than that caused by the Seebeck effect.
As process fluid temperature measurement has become more accurate, there is an increasing need to identify and compensate for smaller and smaller sources of error such as error due to the non-associated voltages. Such error reduction provides more accurate temperature measurements, resulting in more precise process control and the opportunity for increased efficiency and safety.
A process fluid temperature transmitter provides a process fluid temperature output that is substantially independent of any non-associated voltage, thus reducing RTD measurement error. The transmitter includes a plurality of terminals that are couplable to a resistance temperature device (RTD) to form a circuit including at least one non-associated voltage source having a non-associated voltage.